|Mesoscale Convective Thunderstorms in Las Vegas|
and the Rare Heavy Rain Event
Fitzgerald’s casino security guard Juliette Lopez thought this Tuesday would be
like any other. It wasn’t. The day ended with a life or death struggle against a
torrent of water. She won. Barely. Plucked from her flooded car by a newspaper
photographer in the nick of time. Lopez was driving her car in Northwest Las
Vegas in August of 2003 when it began raining …. hard. An intense convective
thunderstorm barely moved as it dumped an estimated 3 inches of rain in about 45
minutes. Flood control channels in the city aren’t designed to hold that much water,
so streets became rivers in the matter of minutes. Lopez was caught in the wrong
place at the wrong time. “I had the window rolled down, and out of nowhere, the
water just starts pouring in. In no time, the water’s covering my seats. It was
moving so fast. I went backwards three blocks in the water,” she told the Las Vegas
Review Journal newspaper. Her SUV crashed into a pole and she was pulled to
safety by the photographer. That day, 9 people had to be rescued from the raging waters
by helicopter, 51 others were plucked from the water by ground crews, thousands
of homes were left without power, freeways were closed, and the Mayor declared a State
Unfortunately, what happened that day happens all too often in Las Vegas.
According to the Regional Flood Control District, since 1960, Las Vegas has
experienced 11 floods that caused at least a million dollars in damage. 31 People have
been killed in Las Vegas in 21 floods dating back to 1960. According to the National
Weather Service office in Las Vegas, there are an average of 127 flood related deaths a
year in the United States. That’s more than hurricanes and tornadoes combined.
Floods can occur during any month in Las Vegas, but the vast majority strike
from July to September, when the Southwestern United States feels the effects of its
monsoon. Monsoon simply means a seasonal shift in predominant air flow and during
these Summer months, Las Vegas receives warm, moist air from the South … from the
Gulf of California and to a lesser extent the Gulf of Mexico and the Pacific Ocean.
Official temperatures in Las Vegas can reach 117° F, creating rising air and thermal
lows. Combine that instable air with a moisture surge from the South, throw in a forcing
mechanism, such as low level convergence or an inverted trough and you have the recipe
for a potentially deadly flooding event.
Predicting thunderstorms can be tricky anywhere, but it becomes much more
difficult when complicated terrain is involved. In Las Vegas, you have a valley bordered
on the North, West and South by mountains. Mt. Charleston, part of the Spring
Mountains to the West, peaks at just under 12,000’ feet. The Sheep Range to the North
has Hayward Peak at just under 10,000’. To the South, the McCollough Range tops out
at about 7,000’. The Las Vegas strip is at about 2,200’ elevation on down to the Las
Vegas Wash on the East side of the valley at 1,720’. Rain on the West side eventually
ends up at the Las Vegas Wash on the East side, usually getting there via miles and miles
of flood control channels.
Forecasting the ingredients for severe thunderstorms and flash flood producing
storms is the easy part. Figuring out where the storms will pop up is next to impossible,
akin to figuring out which kernel of corn will pop first in the frying pan. Trying to
forecast where heavy rain will fall adds another level of difficulty in Las Vegas. Some
of the parameters that focus convective development, such as local convergence zones
associated with gap flow or mountain-valley circulations, surges of low level moisture
from the South, etc…, aren’t sampled very well, therefore they are poorly handled by
model output. There are only a handful of weather stations in and around the Las Vegas
valley, which simply don’t provide enough data to forecast exact locations of mesoscale
events with any certain level of reliability. The nearest radiosonde balloon launch isn’t
even in Las Vegas. They launch about 75 miles to the Northwest in Desert Rock.
To forecast these heavy rainmakers, forecasters here in the valley look for much
of the same stuff as forecasters in other cities: lift, instability and moisture. Lots of
moisture. So, meteorologists look for high precipitable water and dewpoint values.
P.W. values above 1.25” and dewpoint values above 55° F can be enough to produce
Low level moisture is key. If the moisture is located in the mid and upper levels,
much, if not all of the precipitation can evaporate as it falls through the drier lower levels,
leading to a possibly damaging microburst, but not a high rainfall event.
A lot of different tools can be used to assess the heavy rain threat, including
surface observations, soundings from balloons, aircraft reports, satellite imagery, radar,
and a network of 30 automated weather stations operated by the Regional Flood Control
District. This information is used to calibrate model output.
For instability, we look at CAPE, lapse rates and lifted index. People in the
Midwest or South might be surprised by this, but as far as CAPE goes, anything above
1000 j/kg. is enough to get our attention. Any L.I.’s in the negative range can get the job
done. Of course, like elsewhere, none of it gets going if CAP values are too high.
For large scale lift, 500mb to 300mb charts are used to find sources of forcing for
upward motion. For mesoscale lift, forecasters look for convergence zones, remnant
outflow boundaries, thermal gradients and orographic lift. We have something called the
Las Vegas Convergence Zone or LVCZ. It’s created, in part, by differential heating in
the local mountains, which creates a pressure gradient and upslope flow. The flow causes
a valley – mountain solenoid, with rising air on the West side of the valley and
descending air on the East side. This solenoid coupled with a relatively light
Southwesterly to Westerly mean wind flow combine to create the LVCZ. Once morning
inversions are eroded by diurnal heating, the LVCZ can give rise to deep convection and
possibly heavy rain events by destabilizing the air. Storms on the Western and Southern
ends of the valley intensify rapidly in the presence of the LVCZ, while storm
development on the East side of the valley is inhibited by descending air from the LVCZ.
Strong storms on the East side are typically associated with advection, but there are far
fewer storms on the East side compared to the West and South. The LVCZ will migrate
Northeast during the day, guided by the mean wind flow. As it migrates Northeast,
sometimes it’ll be replaced with a secondary convergence zone just Southwest of Las
Vegas. This is primarily generated by gap flow between the Spring Mountains and the
McCollough Range. The LVCZ also provides horizontal vorticity which can be drawn
into convective updrafts, further destabilizing the atmosphere. The head of the Las
Vegas National Weather Service office, Kim Runk, has done extensive research and
written papers on the LVCZ. Among other things, he used output from a RAMS
simulation and drew upon research from several sources (eg. Tyson and Preston-White,
1972; Szoke et. al, 1984; Smolarkiewicz and Rotunno, 1989; Crook et. al., 1990).
It is also critical to identify moisture surge out of the Gulf of California. One of
the main synoptic patterns during the monsoon is an easterly wave over Central Mexico
combined with a strong thermal low over the Southwest. The easterly wave is an
inverted trough, similar to what we see in the tropics. Many of the disturbances in the
easterlies actually originate out of the Gulf of Mexico and not the Gulf of California, as
was the case in August of 2003 when the remnants of tropical storm Erika made their
way through Brownsville, Texas, through Mexico and eventually North into the
Southwestern United States. It added a significant amount of low level moisture which
was a main ingredient in causing the problems discussed in the beginning of this paper.
Gulf surges are hard to identify and track due to the low number of observations
coming out of Northwest Mexico and the Gulf of California. Plus, the synoptic scale
environment is fairly weak. One way to track this surge is by looking for pressure
falls and dewpoint rises in cities between Las Vegas and the Gulf. Cities like Yuma,
Arizona, Blythe and Needles, California and Kingman, Arizona. Also, we look at PW
values. In the August, 2003 event, PW values increased in Guaymas, Mexico from 1.41”
to 1.96” due to the surge.
In conclusion, forecasting for mesoscale convective thunderstorms and the rare
heavy rain event in Las Vegas are a challenge with no end. But with further observation
and study, we can get closer to solving the puzzle.